City-Wide Flood Mitigation Strategy
Responding to Changing Weather Patterns
Like many other municipalities, Edmonton’s weather patterns are changing, and that means our drainage system - a massive network of pipes that collects rainwater - needs to change too.
The City is committed to understanding and taking a proactive approach to dealing with those changes and has initiated the City-wide flood mitigation study.
This study assesses drainage infrastructure in over 160 neighbourhoods mostly built before 1989. Neighbourhoods' drainage systems were built to the standards of their day, but times and needs have changed.
Check out our latest public engagement resources to be involved in this project.
Introduction to the Flood Risk Maps
There were nine preliminary maps released on November 9, 2016 that covered four areas of the City, as shown below. The fifth area is an industrial area in north west Edmonton and will be studied in the future.
Because Edmonton's drainage system was built over time, older parts of the city were built to different standards than what exists now. The Edmonton of decades past, remember, had a much smaller population, with less demand on the system, and experienced different, less-severe rainstorms.
After severe storms in 2004 and 2012 overwhelmed the drainage system, City officials and staff realized that the best way forward would be to start a proactive study of urban flood mitigation measures. This initiated the City-Wide Flood Assessment which produced the flood maps and future project planning to be brought forward to Council on June 9, 2017.
How to read the maps available on Open Data
The preliminary maps give you a sense of how the pipes act during a storm, and what impact the land and roads have on how water might collect in a neighbourhood. To complete the assessment, a large, four-hour rainstorm was modelled over each area shown in each of the nine maps. This type of storm intensity over that large of an area is a worst-case scenario, but it provides the City with important information about how to manage and plan for those types of rainstorms and provide infrastructure solutions that are best suited to each area, if necessary.
Surface Ponding Depths
There are four different colours to show the depth of water that might pool on the ground during a large rainstorm. The colours in the car illustration to the left are green, orange, red and maroon. Green is the least amount of pooling (less than 0.35m) and is acceptable based on City standards. Maroon represents the most significant pooling of water on the ground, at greater than 0.75m. Orange and red are in between.
Surface Ponding Depth
There are solid-coloured lines on the maps. These lines signify the pipes in the area. These pipes are represented by three different colours and show how full the pipes get during a significant rain event. If the pipe is red, it means that the pipes are at capacity. As the pipe fills, the water might start moving up to the manhole and may impact basement flooding.
Green: >2.5 m Below Ground
Yellow: 1.5 m to 2.5 m Below Ground
Red: <1.5 m Below Ground
Surcharge Depth Map
Correction (re: Surcharge Depth Map):
There is an oversight on our part regarding the pipe network published for areas outside the study area (see map above). The overall model creation included a full pipe system in the study areas (excluding Mill Woods) and only the limited large pipe system throughout the rest of the City. These large pipes were used to add a rainfall flows to determine how rainfall moves through the pipe system. These maps are created by running one storm over different areas and taking the worse case for every pipe. Although the map is showing that the pipes are red in Mill Woods this is not correct. The same rainfall has been modeled with a complete pipe model of Mill Woods after the 2012 event and the model indicated that there were no major concerns (such as red pipes) in Mill Woods.
The way our streets drain and water pools during a storm can be impacted by a number of things:
- Whether there are low spots in the neighbourhood where water will run to
- Whether anything is blocking catch basins (examples include people parking over top of them, or leaves or hail creating clogs)
- The amount of water held by pipes in the area
- Major stormwater management facilities in the surrounding area (such as wet ponds and dry ponds)
Anything that influences the way water drains (whether that’s the neighbourhood’s topography or something temporary, like environmental conditions blocking drains) can change the indicated flooding areas on the maps.
All neighbourhoods are different, and how rainstorms occur is unique. This means flooding in each area is unique and not entirely predictable. These maps indicate one possible scenario for rainfall over the city.
This information and more is available online in map format at edmonton.ca/floodmitigation.